Estimate association and dissociation behavior from lab inputs. See affinity, half-life, and target-time instantly. Download clear summaries for your notebook today.
| Scenario | kon (M-1 s-1) | koff (s-1) | KD (nM) | t1/2 off (min) |
|---|---|---|---|---|
| Fast on / moderate off | 1 | 0.002 | 2 | 5.776 |
| Moderate on / slow off | 25 | 0.0002 | 0.8 | 57.76 |
| Slow on / very slow off | 5 | 0.00005 | 1 | 231 |
In many binding assays, kon controls how quickly complexes form at a given ligand concentration. For example, with kon = 1.0×106 M-1 s-1 and [L] = 50 nM, the association term kon·[L] contributes 0.05 s-1. If koff is 0.002 s-1, the observed approach rate is kobs = 0.052 s-1, giving a characteristic time of about 19 s (1/kobs). Faster mixing or higher [L] raises kobs linearly.
Dissociation is first-order in the complex, so koff alone sets stability after washout. A koff of 2.0×10-3 s-1 corresponds to a half-life t1/2 = ln2/koff ≈ 346 s, or 5.8 min, and a residence time τ = 1/koff = 500 s (8.3 min). Cutting koff tenfold extends both metrics tenfold, which is why residence time is often a better comparator than KD in slow-off systems.
Affinity is calculated as KD = koff/kon. Using kon = 1.0×106 and koff = 2.0×10-3 gives KD = 2.0×10-9 M (2 nM). If kon drops to 2.5×105 while koff stays constant, KD rises to 8 nM, even though dissociation kinetics do not change. This separation helps you diagnose whether weaker binding is driven by slower encounter or faster escape.
To estimate time to reach a fraction f of equilibrium, the calculator uses f = 1 − e(−kobs t). At kobs = 0.052 s-1, reaching 90% requires t = −ln(0.1)/0.052 ≈ 44 s. Reaching 99% needs about 88 s because −ln(0.01) doubles. These numbers guide incubation times and show diminishing returns past ~3–5 time constants.
Keep rates in M-1 s-1 and s-1, and convert concentrations to molar before combining terms. A common mistake is entering 50 nM as 50 M, which inflates kon·[L] by 109. Report kon, koff, and the derived KD with units, plus the concentration used for kobs. Exported CSV and PDF summaries support consistent notebooks, methods sections, and cross-experiment comparisons. In practice.
kon describes how quickly ligand and target form a complex per molar concentration. koff describes how quickly the complex breaks apart. Together, they define affinity and time behavior.
For a simple 1:1 interaction at equilibrium, the ratio of dissociation to association rate constants equals the equilibrium dissociation constant. This links kinetic measurements to an equilibrium affinity metric.
kobs is the observed first‑order approach rate at a specific ligand concentration: kobs=kon·[L]+koff. Use it to plan incubation times or compare conditions at the same [L].
Higher target fractions require disproportionately more time. Going from 90% to 99% doubles the needed time because the formula uses −ln(1−f). This highlights diminishing returns in long incubations.
Enter the numeric value and choose the correct unit (M, mM, µM, nM, pM). The calculator converts to molar internally before computing kobs and timing outputs.
After binding has occurred, dissociation follows first-order kinetics with rate koff. Half-life is ln2/koff, and residence time is 1/koff. Both reflect how long the complex persists.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.